An ordinary microscope can disclose detail where the transparency of the object varies. Many biological objects, however, are nearly completely transparent, and consequently almost invisible to the classical microscope. Phase contrast, discovered by Frits Zernicke in 1932, made possible the observation of such objects by making visible small differences in optical path even though the objects studied were quite transparent.
In many cases, however, visualization is not enough; some measure of the optical path through an object is desirable. Examples are the determination of the depth of grooves in a transparent object, or measuring amounts of food injested by a living cell subjected to a particular stimulus. In order to make such measurements, the phenomenon of interference has been used. Two light beams are made to interfere, each having been affected in different ways by their passage through the microscope. In order to obtain high resolving power, a microscope must utilize a very divergent beam of light in the neighborhood of the object. This greatly restricts the design of an interferometer since in the absence of the object, the two beams must have nearly identical path lengths. Several types of interference microscopes have nevertheless been developed. Perhaps the simplest in principle is that in which there are twin microscopes, one with the object and one without. The light is split into two beams. One beam is sent through each microscope, the beams are recombined and the resulting interference fringes are observed. This system, the Mach-Zehnder, has been known since the beginning of the century, but has proven difficult to build and to adjust. Other systems exist in which the two beams traverse the specimen either in different planes of focus or in slightly different transverse paths. All of these yield interference fringes proportional to differences in adjacent optical paths, and so are sensitive, not to the actual optical path, but to its spatial derivative. Such methods often yield very beautiful detail, but are not adapted for quantitative measurements of the amount of material in the light path. There is thus need for an instrument which can yield a true measure of optical thickness and yet be more practical and also more adaptable to existing microscope designs.